"We are now facing an existential threat to the security of the United States of America," said McCain.

But nothing that happens in Mesopotamia is going to threaten the existence of the United States. As for the terrorist threat from ISIS, for us it is neither greater nor less than it was a week ago.

The existential threat here is to Iraq. Its survival as one nation is now in question, with the possibility it could be torn apart in a civil and sectarian war. But this is preeminently Iraq's problem, not ours.

And if Prime Minister Nouri al-Maliki, his 900,000-man army, and Shia militia cannot defend Baghdad from a few thousand Islamist warriors, America is under no obligation to do it for them.

Maliki told us to go home three years ago. We did. And before we plunge back into that misbegotten war, let us consider what the real threats are – to America.

The Islamic State of Iraq and Syria consists of fanatics who seek to carve a caliphate out of territory they now control from Aleppo in Syria to 60 miles north of Baghdad.